Field of the Invention
The present invention relates to an electronic module, and more specifically, to an electronic module including a multilayer board into which a core and a coil are assembled.
Description of the Related Art
A DC-DC converter is known as a type of electronic module formed by mounting various electronic components on a board. In a commonly-known DC-DC converter, a transformer and an inductor which are core/coil components formed by combining a core composed of a magnetic material and a coil are surface-mounted on a board.
In recent years, thickness and size reductions have been a critical issue in equipment into which a DC-DC converter is assembled. Accordingly, DC-DC converters are also desired to be reduced in size.
Hence, attempts are being made to reduce the height of core/coil components, among electronic components to be mounted on a DC-DC converter, which are components comparatively large in height. Here, a transformer which adopts a laminated coil formed by stacking a plurality of thin laminar sheet coils is available as a transformer the height of which is kept low. As such a transformer, there has been known a transformer mounted on an electronic circuit device shown in, for example, Japanese Patent Laid-Open No. 2012-134291.
In the transformer disclosed in this Japanese Patent Laid-Open No. 2012-134291, an electrical conductor arranged in each laminar sheet coil is electrically connected through a connection hole wiring (through-hole wiring or via wiring) buried in a connection hole created in each laminar sheet coil.
Incidentally, attempts are being made to directly assemble such a transformer as mentioned above into the multilayer board of a DC-DC converter, in order to cope with a further decrease in thickness. Specifically, a core insertion hole to insert a core through is arranged in a predetermined region of the multilayer board serving as a base of the DC-DC converter. Then, a coil wiring pattern to serve as part of the coil is formed in each layer, so as to surround this core insertion hole. In addition, a coil wiring pattern in each layer is electrically connected through a connection hole wiring, i.e., a through-hole wiring or a via wiring, buried in a connection hole arranged in each layer, as in the transformer disclosed in Japanese Patent Laid-Open No. 2012-134291. The coil is thus formed in the multilayer board. In addition, a transformer is formed in the multilayer board as the result of the core being inserted through the abovementioned core insertion hole. In this case, dimensions in the thickness direction of the board are further reduced since the transformer is directly assembled into the multilayer board, thereby contributing to thinning the DC-DC converter.
In the multilayer board into which such a transformer as described above is assembled, through-hole wirings and the like for connecting the coil wiring patterns of respective layers are generally formed in a region where other electronic components are to be mounted. Other electronic components are mounted so as to avoid trespassing on these through-hole wirings.
Incidentally, there are also demands for the size reduction of DC-DC converters and the high densification of electronic components to be mounted on the converters. If through-hole wirings and the like are present in a region where the electronic components are mounted as described above, however, an area of a board surface available for the electronic components to be mounted decreases, thus making it difficult to meet these demands.
Hence, so-called edge-face through-holes are formed on inner wall surfaces of the core insertion hole, in place of the abovementioned through-hole wirings and the like, to electrically connect the coil wiring patterns of respective layers using these edge-face through-holes. This method does not disturb the mounting of other electronic components, and therefore, contributes to the size reduction and high-density packaging of DC-DC converters.
The edge-face through-holes face the core, however, and therefore may come into contact with the core to cause short-circuiting. In addition, the inner wall surfaces of the core insertion hole are curved so as to be conformal to the outer shape of the column-shaped core. If the edge-face through-holes are formed on such curved surfaces as described above, burrs are liable to arise in the course of forming the holes. If any burrs containing portions of the conductor of the edge-face through-holes, short-circuiting is more likely to occur.
An object of the present invention, which has been accomplished in view of the above-described circumstances, is to provide an electronic module capable of preventing short-circuiting between a core and an edge-face through-hole.